The present invention relates to a "dry" objective for a microscope, the object being separated from the objective by a layer of air.
The objective of the present invention gives an image of infinity and is used with a microscope provided with a convergent lens to form an image in the focal plane of the ocular. The objective need not be positioned precisely.
The objective of the present invention is used with a microscope for the examination of transparencies or with a metalographic microscope.
In a microscope the limit of resolution varies in accordance with 1/n sin u, n being the index of refraction of the medium separating the object from the objective (air in the case of a "dry" objective, and then being almost equal to 1), u being the angular half opening of the objective seen from the object. The objectives of a microscope are characterized by a number, called the numerical opening, which is equal to n sin u, more particularly to sin u for a "dry" objective. The aberrations, that is, the defects altering the quality of the formed images, increase if it is desired to increase the numerical opening to increase the power of resolution. A microscope objective having a large numerical opening should be corrected for various aberrations.
A microscope objective should thus be corrected for chromatic aberration. It is known that the index of refraction for refringent materials, usually making up the lenses of the objective, varies with the wave length. If the object point is a source of white light (super-position of an infinity of monochromatic radiations), the objective gives an infinity of monochromatic images distributed along the axis. It is known that a simple convergent and divergent system provides inverse distributions of the monochromatic images. The chromatic aberrations can be corrected by suitable choice of the glasses of the lenses.
An optical glass is characterized by the parameter .gamma. = n.sub.d - 1/n.sub.F - n.sub.c, known as the dispersive power of Abbe number, n.sub.F and n.sub.c corresponding to the indices of refraction measured for the rays C and F (red and blue) of hydrogen, nd corresponding to the index measured for the ray d of helium, the wave length of which is intermediate between those of rays C and F. The average index nd corresponds to that of the glass. The partial dispersion is then defined as .gamma. p - n.sub.d - n.sub.c /n.sub.F - n.sub.c.
A convergent lens is characterized by a certain distance between the images obtained for the rays C and F. A divergent lens is characterized by a certain distance between the images obtained for the same rays C and F. When chromatism is corrected, the images corresponding to the rays C and F are made to coincide by associating a divergent lens and a convergent lens for which the glasses are suitably chosen. When the correction is obtained, the images obtained for the rays C and F being substantially superposed, the images obtained for the other radiations (ray d, for example) remain separated from the proceeding images. This is spoken of as secondary chromatism. An objective corrected for secondary chromatism is called apochromatic. To reduce secondary chromatism of a doublet formed of a convergent lens cemented to a divergent lens, it is necessary that the partial dispersion V.sub.p of the glasses be substantially equal.
A microscope objective should also be corrected from the point of view of curvature of field. An optical system is said to present the aberration of curvature of field when the point object, describing a plane perpendicular to the axis, the location of the image describes a surface of revolution which is offset from the plane of the ideal image. The curvature of field of an assembly of lenses is measured by the Petzval sum which, in the case of thin lenses, is equal to the sum of the quantities 1/nF calculated for the different lenses of the assembly, F representing the focal distance and n representing the index of refraction of the glass. It is known that the higher the indexes, the closer the Petzval sum moves towards 0 and the smaller the curvature of the field and inversely. Further, a lens of negative power (a divergent lens) has a Petzval sum which is negative which decreases the total value of the Petzval sum.
An objective of a microscope should also be corrected from the point of view of spherical aberration. This purely geometrical aberration which is produced in monochromatic light, occurs by reason of the fact that the marginal rays are more deviated by a simple convergent system than the central rays, a divergent system giving an aberration in opposite sense.
A microscope objective should be more generally corrected from the point of view of stigmatism, which condition occurs when all rays issuing from a point object pass through the point image.
An apochromatic microscopic objective has already been realized, that is, having a reduced secondary chromatism. This result has been obtained usually by the use of fluorspar crystals, of low dispersion, to construct certain elements of the objective. This type of objective has the inconvenience of being relatively burdensome.
Microscope objectives corrected from the point of view of curvature of field are known. An objective of this type is described in French Pat. No. 1,310,259.
The present invention has for its object to provide a microscope objective having focal lengths between 2.5 and 8mm. It has a large numeric opening on the order of 0.75. The objective is provided with a plane field, the Petzval sum being small with respect to that of an objective of equivalent focal length. The objective has a reduced secondary chromatism and is perfectly corrected from the point of view of spherical aberration, coma, and astigmatism. The distance between the object and the first element of the objective is large. In an embodiment of the objective the use of fluorspar for predetermined lenses provides a particularly accurate correction for secondary chromatism.